1. Field of the Invention
This invention relates to an illumination system and method for imaging thermosensitive or light-sensitive media such as printing plate precursors. More particularly, this invention is directed to an illumination system in which laser light is conveyed to a modulator, and the modulated laser light is passed through separating means to permit only the passage of zero order radiant energy which is employed to image the media.
2. Background Information
The present invention concerns the production of images on thermosensitive supports requiring high radiant energy although not as high as the energy required for the ablation of a special coating as described, for example, in copending International Patent Application No. PCT/US01/40002 entitled xe2x80x9cHIGH POWER LASER HEAD SYSTEM,xe2x80x9d incorporated herein by reference.
Systems producing a large number of individual image-forming rays are of considerable interest for speed and performance in flat bed systems in which bands comprised of a large number of pixels are produced by a translating optical projection unit. The present invention makes it possible to transfer to an image receiving media most of the intensity produced by a laser bar having an elongated line of emitters. It combines the use of new commercially available laser sources and a special optical arrangement of components located upstream and downstream of a total internal reflection (TIR) light modulator.
The use of a line of several light sources to illuminate a TIR modulator is not new, as shown for example, in U.S. Pat. No. 4,281,904, which is incorporated herein by reference. In this patent, the image of each source falls on a screen having several portions, as shown in FIG. 6e of this patent, allowing passage of superior diffraction orders and excluding low orders. Another example is described in U.S. Pat. No. 5,517,359, incorporated herein by reference, which discloses the use of lenslets to direct the rays emerging from separate emitters to a modulator.
Additional illumination systems are disclosed in U.S. Pat. No. 6,137,631, U.S. Pat. No. 6,222,666 and U.S. Pat. No. 6,369,936, all of which are incorporated herein by reference.
To increase the effectiveness of a TIR system for applications in which the intensity is more important than contrast, the arrangement in which diffracted rays of zero order are allowed to reach the radiation sensitive media at the exclusion of the others is preferred. The present invention makes it possible to couple to a TIR modulator an elongated source of radiation comprising, for example, a laser bar provided with several laser emitters, regardless of the repartition of these emitters along the bar and their spacing. More specifically, the invention makes it possible to use laser diodes among the most powerful presently available, with a filling factor coefficient xe2x80x9cffxe2x80x9d (width of emitters over width of the bar) of over 50%, including for example 60-watt diodes from such suppliers as Opto-Powers, Coherent Technologies and Siemens Corp. The configuration of these diodes, partially because of their high xe2x80x9cffxe2x80x9d number, precludes the effective use of microlens arrays, as presently known in the art, to merge emitted beams as will be explained herein.
Therefore, it is an object of the present invention to provide a high power electro-optical modulating system excluding the use of a micro-lens array.
An optical system for directing the radiant energy produced by a bar of high power laser emitters to a TIR spatial modulator from which selected rays are directed to a media requiring radiation of high radiant intensity comprises:
(a) a plurality of laser radiation sources such as a laser bar having a plurality of emitters;
(b) means for collimating such as a collimating lens for collimating at least a portion of the rays provided by the laser radiation sources;
(c) first imaging means such as a plurality of lenses for providing a first image of the laser radiation sources;
(d) means for collimating such as a collimating lens and means for focusing such as a cylindrical lens for collimating and focusing the rays from the first image;
(e) means for modulating such as a total internal reflection modulator for modulating rays from the first image;
(f) second imaging means such as a plurality of lenses for providing a second image of the modulated rays;
(g) means for separating such as a plate having a slit for separating rays from the diffraction orders of the modulated second image to permit the passage of only zero order rays; and
(h) means for imaging such as a plurality of lenses for imaging the zero order rays.
The system may include an optical mixer in the form of a glass blade to direct the radiated rays from the emitters of the laser bar to the modulator.